Acellular pertussis vaccines have reduced considerably the systemic reactions observed with cellular vaccines of DTP but have not eliminated the extensive swelling (sometimes involving an entire limb) observed after the fifth injection of DTaP. This reaction, an Arthus hypersensitivity due to high levels of antibodies reacting with DTaP, could discourage its use in adults who serve as the major reservoir of pertussis for infants. A critical level of pertussis toxin (PT) antibodies is both essential and sufficient to prevent infection with Bordetella pertussis. We point out the similarities between the immunity induced by diphtheria and pertussis toxoids. The genetically-inactivated pertussis and diphtheria toxins are more immunogenic and therefore induce comparable levels of anti-toxin at lower protein levels than the formalin-treated native toxins. In addition the chemically inactivated pertussis toxoids exert mild but constant immunosuppressive activity. Replacement of the D and aP components with these improved antigens will reduce the amount of protein in DTaP and, most likely, the incidence and severity of local reactions in teenagers and adults.